The present invention relates to electrolytic cells employing an oxygen cathode which are used for, e.g., sodium chloride electrolysis by the ion-exchange membrane method. More particularly, the invention relates to electrolytic cells employing a gas diffusion electrode as an oxygen cathode which can be improved in any of the following: a caustic solution can be effectively fed and discharged; caustic solution leakage through the gas diffusion electrode into the gas chamber can be effectively and appropriately coped with; a caustic chamber serving as an electrolytic solution passageway can be constituted so as to have an exceedingly small thickness; oxygen gas can be evenly fed to and discharged from the gas chamber having the gas diffusion electrode; a gas- and liquid-permeable gas diffusion electrode is used as the gas diffusion electrode to thereby enable a stable electrolytic operation to be continued at a high current efficiency; and power distribution in the electrolytic cell employing a gas diffusion electrode can be conducted so as to apply a voltage to a large area without considerably modifying the structure of a conventional electrolytic cell.
An electrolytic cell employing an anode, an ion-exchange membrane, and an oxygen cathode comprising a gas diffusion electrode has hitherto been proposed for use in sodium chloride electrolysis or Glauber""s salt electrolysis.
In such a conventional electrolytic cell employing a gas diffusion electrode, e.g., an electrolytic cell for sodium chloride electrolysis, the electrolytic cell is constituted of elements including a cathode element, cathode collector frame, and caustic chamber frame and these elements have been assembled together with gaskets interposed therebetween. A caustic solution is fed and discharged through liquid inlets and outlets of a caustic chamber disposed in the cathode element. Since this electrolytic cell has the constitution described above, it necessitates gaskets for assembly.
Because of this, this electrolytic cell has a complicated structure and has had a problem that there is a high possibility that the caustic solution might leak out due to a decrease in sealing properties in the joints between members, e.g., in the gaskets.
This electrolytic cell has further had a problem that although there is a possibility that the caustic chamber of the cathode element might suffer electrolytic corrosion, it is difficult to plate the caustic chamber with a metal having resistance to corrosion by NaOH, e.g., silver, for corrosion prevention because the chamber has a complicated structure.
Furthermore, in the conventional ion-exchange membrane type electrolytic cell for sodium chloride electrolysis, in the case where a gas diffusion electrode is used as an oxygen cathode in place of the gas generation type cathode, a gas diffusion electrode which is liquid-impermeable is usually employed to constitute the electrolytic cell so as to have three chambers. In such a case, since the electrolytic cell for practical use has a height of 1.2 m or higher and the solution chamber thereof is filled with an electrolytic solution, a high fluid pressure attributable to the electrolytic solution is applied to a lower part of the gas diffusion electrode and this is causative of liquid leakage from the catholyte chamber to the gas chamber.
When a gas diffusion electrode is attached to such a vertical electrolytic cell and an electrolytic solution is fed thereto, then a difference in fluid pressure results. Namely, a high fluid pressure is applied to a lower part of the gas diffusion electrode as stated above, whereas almost no fluid pressure is applied to an upper part. This difference in fluid pressure is causative, in the lower part, of liquid leakage from the catholyte chamber to the gas chamber, and is causative, in the upper part, of gas leakage through the gas diffusion electrode to the electrolytic solution side.
Furthermore, when an actual electrolytic operation is conducted under such conditions that the fluid pressure is higher than the gas pressure for the gas diffusion electrode, then a large amount of the electrolytic solution (caustic solution) leaks out into the gas chamber in the case where the gas diffusion electrode has low water resistance and the sealing is insufficient. There has hence been a problem that this leakage inhibits gas feeding and reduces the electrode performance and electrode life.
In particular, gas diffusion electrodes having low water pressure resistance have limited uses.
In addition, if the gas chamber is filled with a caustic solution, this caustic solution further flows into a lower gas chamber for gas discharge or feeding (which has conventionally been formed in the frame of the electrolytic cell). In this case, since the lower gas chamber is corroded by the caustic solution, the inner surface of the lower gas chamber should be plated beforehand with a metal having resistance to corrosion by NaOH, e.g., silver. In the conventional electrolytic cell, however, it has been difficult to subject the inner surface of the lower gas chamber to corrosion-preventive plating because of the structure thereof. There has been a further problem that although the cathode collector frame has been sealed to the lower gas chamber with a gasket, insufficient sealing permits the caustic solution to flow into the cathode element and corrode the inside of the element. Furthermore, in some electrolytic cells, it has been difficult to attach a gas chamber to the existing cathode element due to the structure of the element.
Many of the gas diffusion electrodes for use in such electrolytic cells are usually composed of two layers, i.e., a reaction layer for subjecting a liquid reactant to an electrolytic reaction and a gas feed layer which is permeable to gases but impermeable to the electrolytic solution.
The reaction layer is constituted of a hydrophilic carbon black having a catalyst supported thereon, a hydrophobic carbon black, and polytetrafluoroethylene (PTFE). The reaction layer is produced by dispersing and self-organizing those materials in various proportions so as to form hydrophilic areas into which an electrolytic solution penetrates and hydrophobic areas to which a gas is fed. The reaction layer thus produced has been attached to a cell and used either as it is or after only the surface thereof is hydrophilized by adhering fine hydrophilic particles to the surface.
Moreover, a technique has been used in which a structure having through-holes and a high porosity is interposed between an ion-exchange membrane and the reaction layer of a gas diffusion electrode in order to secure electrolytic solution passageways between the ion-exchange membrane and the reaction layer of the gas diffusion electrode.
As a result, flows of an electrolytic solution have been secured. However, there has been a problem that the caustic chamber serving as a cathode chamber into which an electrolytic solution is to be introduced has an increased thickness and inevitably has increased electrical resistance and this necessitates use of a higher voltage.
With respect to a gas chamber having a gas diffusion electrode, it has conventionally been known that there is a relationship in which the higher the linear velocity of the oxygen which is in contact with the gas diffusion electrode serving as an oxygen cathode, the higher the rate of diffusion of the oxygen into the electrode.
Because of this, investigations have been made on: a technique for providing a gas chamber formed by press-molding a nickel sheet to form in a central part thereof a depression having the same size as a gas diffusion electrode, using the depression and the gas diffusion electrode to form a gas chamber, inserting into the chamber a nickel mesh serving as a spacer for securing oxygen passageways to constitute a gas chamber for the gas diffusion electrode and thereby form an exclusive gas chamber, forming in this gas chamber a space which enables oxygen to have a linear velocity necessary for sufficient diffusion into the electrode, and further forming a structure which enables oxygen to come into even contact with the gas diffusion electrode; and a gas chamber which is formed by silver-deposited ridges of a metal plate having ridges and grooves and a gas feed layer of a gas diffusion electrode and is produced by bonding the silver present on the ridges of the grooved metal plate with the gas diffusion electrode by hot pressing to thereby use the grooves of the metal plate as gas passageways.
However, these gas chambers having a diffusion electrode each relates to a technique for accelerating oxygen diffusion in the gas chamber and making the diffusion even. There has been an unsolved problem that the even feeding of oxygen gas into a gas chamber and the even discharge thereof are not taken in account at all.
Furthermore, brine electrolysis with a conventional gas diffusion electrode is disadvantageous with respect to the deterioration of the gas diffusion electrode or the recovery of the caustic soda yielded. This electrolysis has had a drawback that long-term operation is impossible or the caustic soda penetrates into the anode chamber to reduce the current efficiency.
An electrolytic cell employing a gas- and liquid-permeable gas diffusion electrode has been proposed as a means for eliminating that drawback (see, for example, Unexamined Published Japanese Patent Application No. 7-126880). In this invention, the concentrated aqueous caustic soda solution which is being yielded is prevented from remaining around the interface between an ion-exchange membrane and a gas diffusion electrode and penetrating through the ion-exchange membrane to the anode chamber side, by using a gas- and liquid-permeable gas diffusion electrode as the gas diffusion electrode. As a result, the caustic soda which is being yielded can be permitted to pass through the gas diffusion electrode to the cathode chamber side and be easily recovered. Thus, the current efficiency in caustic soda generation can be kept high and the anode chamber members having poor alkali resistance can be protected.
However, this electrolytic cell is slightly unsatisfactory in current efficiency and the stability of electrolytic operation, because water and oxygen gas are fed through a substrate, e.g., a porous sheet, to the gas diffusion electrode, which is a material obtained by kneading a carbonaceous material and PTFE, while feeding a dilute aqueous solution of caustic soda and an oxygen-containing gas to the cathode chamber through feed openings. In addition, there has been a problem that the existing cathode frame should be modified and the modification cost is high.
With respect to methods of power distribution in electrolytic cells employing a gas diffusion electrode, the conventional methods of power distribution in electrolytic cells employing a gas diffusion electrode, i.e., methods for the attachment of a gas diffusion electrode and for power discharge, are roughly divided into the following two types.
(1) Power Supply through Periphery of Gas Diffusion Electrode
The peripheral dimensions of a gas diffusion electrode are regulated so that the periphery of the gas diffusion electrode slightly overlaps the gasket-sealed areas of a cathode element or cathode collector frame (pan or plate form). The periphery of this gas diffusion electrode is brought into contact with the gasket-sealed areas of the cathode element or cathode collector frame. A gasket is placed thereon, and the whole electrolytic cell is assembled and fastened, whereby the contact areas also are fastened. In this method, a current is permitted to flow from these fastened areas.
(2) Cathode Collector Frame-Gas Diffusion Electrode Integration
A catalyst layer of a sheet-form gas diffusion electrode is placed on a metal gauze which is for use in a gas chamber and has been attached to a cathode collector frame. This assemblage is pressed with a pressing machine at a high temperature and a high pressure to sinter the catalyst and simultaneously unite the metal gauze for a gas chamber with the catalyst layer. In this method, power is thereby discharged to the cathode collector frame and cathode element through the gas diffusion electrode.
However, such conventional methods for the attachment of a gas diffusion electrode and for power discharge have had the following problems due to their actions and functions
(a) Power Supply through Periphery of Gas Diffusion Electrode
In small electrolytic cells, an appropriate conduction area can be secured. However, in practical electrolytic cells having a reaction area (electrode area) of 3 m2, a sufficient conduction area cannot be secured and this part has increased contact resistance. Furthermore, in large electrolytic cells, the sides of the reaction area each has a length of at least 1 m. Even when the gas diffusion electrode contains a conductor therein, this conductor has high electrical resistance, i.e., the structure has increased resistance. The operation of such large electrolytic cells is hence inferior in profitability. In addition, in the case where a gas diffusion electrode having low strength is used and pressed with a gasket, the gas electrode breaks in the pressed parts to cause leakage of oxygen and caustic soda solution through these parts.
(b) Cathode Collector Frame-Gas Diffusion Electrode Integration
Since practical electrolytic cells have a reaction area of about 3 m2, integration of a gas diffusion electrode with a cathode collector frame necessitates a huge pressing machine and pressing mold and is uneconomical.
Furthermore, even when a gas diffusion electrode and a cathode collector frame are united with each other, the assembly of these having a size as large as 3 m2 has an exceedingly small thickness for the size and is flimsy. Consequently, the assembly has considerably low strength and, hence, it is exceedingly difficult to transport it from the pressing factory to a place where an electric cell is to be assembled. This is a problem common also to the method of xe2x80x9cPower Supply through Periphery of Gas Diffusion Electrodexe2x80x9d described above.
Moreover, in the case where the gas diffusion electrode is replaced with a fresh one, it is difficult to remove the catalyst layer from the collector frame. It is hence necessary to finally replace the whole collector frame with a fresh one, and this is uneconomical.
The invention has been achieved in view of such conventional problems. An object of the invention is to provide an electrolytic cell which employs a gas diffusion electrode and has a simple structure and in which a conventional electrolytic cell can be used as it is and a chamber capable of being easily subjected to corrosion-preventive metal plating can be used to completely prevent the leakage of caustic solution.
Another object of the invention is to provide an electrolytic cell in which a lower gas chamber is disposed at the lower outer edge of the cathode element, whereby caustic solution leakage through a gas diffusion electrode into a gas chamber can be effectively and appropriately coped with.
Still another object of the invention is to provide an electrolytic cell which employs an oxygen cathode and in which the thickness of a caustic chamber is reduced as much as possible to thereby attain a reduced energy loss and a reduced voltage.
A further object of the invention is to provide an electrolytic cell in which chambers having many holes for oxygen gas feed and discharge are attached to a cathode collector frame to thereby enable oxygen gas to be evenly fed to and discharged from a gas chamber having a gas diffusion electrode.
A still further object of the invention is to provide a constitution in which oxygen gas can be evenly fed to and discharged from a gas chamber having a gas diffusion electrode without modifying the structure of a conventional electrolytic cell.
A still further object of the invention is to provide an electrolytic cell in which water and oxygen gas are directly introduced into a conductive porous material which is a gas chamber component disposed between a gas diffusion electrode and a cathode collector frame and used for power supply to the gas diffusion electrode, whereby a higher current efficiency and a more stable electrolytic operation can be continued.
A still further object of the invention is to provide a method of power distribution in an electrolytic cell employing a gas diffusion electrode, which can be speedily carried out at low cost without necessitating a modification of an existing cathode element at all.
According to the invention, those objects of the invention are accomplished specifically by the following means.
1. An electrolytic cell employing an anode, an ion-exchange membrane and an oxygen cathode comprising a gas diffusion electrode, characterized in that a caustic chamber frame comprising an upper chamber, as caustic solution discharge openings, and a lower chamber, as caustic solution introduction openings, which are connected to each other through caustic solution passageways is disposed at outer edges of the electrolytic cell which comprises: a gas chamber having oxygen gas outlets and inlets for the gas diffusion electrode which meet upper-and lower-chamber oxygen gas outlets and inlets formed on the center side of and adjacently to a cathode element along the plane of a cathode collector frame; and a cathode chamber which is the space between the gas diffusion electrode and the ion-exchange membrane and into which a caustic solution is to be introduced.
2. The electrolytic cell described in item 1 above, characterized in that the caustic solution passageway from each chamber is formed between parallel plate materials having a narrow gap and has spacers disposed therein at an interval of from 10 to 100 mm for the purposes of evenly dispersing a caustic solution and securing strength.
3. An electrolytic cell employing an anode, an ion-exchange membrane and an oxygen cathode comprising a gas diffusion electrode, characterized in that, in the electrolytic cell comprising: a gas chamber having oxygen gas feed openings for the gas diffusion electrode, the oxygen gas feed openings being connected to an oxygen gas feed part of a cathode element; and a caustic chamber which is the space between the gas diffusion electrode and the ion-exchange membrane and into which a caustic solution is to be introduced, a lower gas chamber is disposed as a gas discharge part under the gas chamber at the lower outer edge of the cathode element along the plane of a cathode collector frame.
4. An electrolytic cell employing an anode, an ion-exchange membrane and an oxygen cathode comprising a gas diffusion electrode, characterized in that a thin nickel frame having, in its upper and lower frame parts, caustic solution passage holes which meet caustic solution outlets and inlets of caustic chambers disposed in an upper and lower part of a cathode chamber frame, a thin nickel frame having comb-like slits in its upper and lower frame parts, and a thin nickel frame having no holes in its upper and lower frame parts are disposed in this order toward the ion-exchange membrane to constitute a caustic chamber frame and thereby constitute a caustic chamber having an exceedingly small thickness.
5. The electrolytic cell described in item 4 above, characterized in that the nickel frames are tightly sealed to each other with a sealing material or the nickel frames are united together by means of laser welding.
6. An electrolytic cell employing a gas diffusion electrode, characterized in that an upper gas chamber for oxygen gas introduction and a lower gas chamber for oxygen gas discharge are disposed on the inner side of a cathode element along the plane of a cathode collector frame so that the upper and lower gas chambers meet gas outlets and inlets formed in the upper and lower edges of a gas chamber having the gas diffusion electrode.
7. An electrolytic cell employing a gas diffusion electrode, characterized in that a gas- and liquid-permeable gas diffusion electrode is used as the gas diffusion electrode, and that an upper chamber connected to a gas chamber having the gas diffusion electrode and a lower chamber connected to the gas chamber are disposed along the plane of a cathode collector frame of a cathode. element on the upper and lower edges thereof to thereby respectively constitute a part for feeding oxygen gas and water and a part for discharging gas and caustic solution.
8. A method of power distribution in an electrolytic cell employing a gas diffusion electrode, characterized in that an oxygen cathode constituted of a gas diffusion electrode, a gas chamber and a cathode collector frame is disposed so that the cathode collector frame of the oxygen cathode faces a meshed metallic material of a cathode chamber frame conductor of a cathode element and a necessary planar pressure is maintained with a gas pressure to bring the cathode collector frame into contact with the meshed metallic material and electrically connect these.